This invention relates to a blown tube process and apparatus for producing biaxially oriented synthetic thin gauge resin film such as polypropylene film, and more particularly to a blown tube apparatus where the extruder die, reheat ovens, and hot air ring are caused to rotate and oscillate in unison to provide a flatter, more uniform polypropylene film.
In a well-known biaxially oriented blown tube apparatus and process, polypropylene resin is fed through an extruder die head to form a continuously moving cooled and solidified tubular section or stalk which then passes through a reheat furnace to bring it up to a predetermined orientation temperature. At the predetermined orienting or softening temperature, the stalk is insufflated or expanded into a very large thin gauge cylindrical structure referred to as a blown tube or bubble. This cylindrical structure is closed at one end by having it pass through a pair of nip rolls, and thereafter the double lay flat biaxially oriented section emanating from the nip rolls is slit and wound on storage or mill rolls.
One of the problems of such a process is that an irregular section or a gauge non uniformity causes a continuing build up in the film in a mill roll because the non uniformity is continually wound on itself in each turn. Large rolls so wound have differences in roll diameters because of this cascading effect. These differences, because of the creep characteristics of the film degrade the basic flatness eveness and straightness of the film. When precision winding machines such as capacitor roll winding machines are utilized to wind capacitor rolls, this degradation causes skewering and wrinkling in the winding film which are extremely undesirable factors.